Short-wave infrared super-continuum lasers for natural gas leak detection, exploration, and other active remote sensing applications

ABSTRACT

A measurement system performs non-destructive quality control or constitutive analysis using a light source generating an output optical beam using semiconductor sources that generate an input optical beam, a multiplexer configured to receive the input beam and to form an intermediate optical beam, and fibers configured to receive the intermediate beam and to form the output beam. The fibers include a fused silica fiber having a core diameter less than 400 microns. The output beam includes wavelengths between 700 and 2500 nanometers and has a bandwidth of at least 10 nanometers. The system includes a measurement apparatus that receives the output beam and delivers the beam to a sample to generate a spectroscopy beam. A receiver is configured to receive the spectroscopy beam and process the beam to generate an output signal. The light source and receiver are remote from the sample, which includes plastics or food industry goods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/357,225filed Nov. 21, 2016, which is a continuation of U.S. application Ser.No. 14/650,981 filed Jun. 10, 2015, now U.S. Pat. No. 9,500,634, issuedNov. 22, 2016, which is the U.S. national phase of PCT Application No.PCT/US2013/075767 filed Dec. 17, 2013, which claims the benefit of U.S.provisional application Ser. No. 61/747,485 filed Dec. 31, 2012, thedisclosure of which is hereby incorporated by reference in its entirety.

This application is related to U.S. provisional application Ser. No.61/747,472 filed Dec. 31, 2012; U.S. provisional application Ser. No.61/747,477 filed Dec. 31, 2012; Ser. No. 61/747,481 filed Dec. 31, 2012;Ser. No. 61/747,487 filed Dec. 31, 2012; Ser. No. 61/747,492 filed Dec.31, 2012; Ser. No. 61/747,553 filed Dec. 31, 2012; and Ser. No.61/754,698 filed Jan. 21, 2013, the disclosures of which are herebyincorporated in their entirety by reference herein.

This application has a common priority date with commonly owned U.S.application Ser. No. 14/650,899 filed Jun. 10, 2015, which is the U.S.national phase of International Application PCT/US2013/075700 entitledNear-Infrared Lasers For Non-Invasive Monitoring Of Glucose, Ketones,HBA1C, And Other Blood Constituents; International ApplicationPCT/US2013/075736 entitled Short-Wave Infrared Super-Continuum LasersFor Early Detection Of Dental Caries; U.S. application Ser. No.14/108,995 filed Dec. 17, 2013 entitled Focused Near-Infrared Lasers ForNon-Invasive Vasectomy And Other Thermal Coagulation Or OcclusionProcedures; U.S. application Ser. No. 14/108,986 filed Dec. 17, 2013,now U.S. Pat. No. 9,164,032 entitled Short-Wave Infrared Super-ContinuumLasers For Detecting Counterfeit Or Illicit Drugs And PharmaceuticalProcess Control; U.S. application Ser. No. 14/108,974 filed Dec. 17,2013 entitled Non-Invasive Treatment Of Varicose Veins; and U.S.application Ser. No. 14/109,007 filed Dec. 17, 2013 entitledNear-Infrared Super-Continuum Lasers For Early Detection Of Breast AndOther Cancers, the disclosures of which are hereby incorporated in theirentirety by reference herein.

BACKGROUND AND SUMMARY

Remote sensing or hyper-spectral imaging often uses the sun forillumination, and the short-wave infrared (SWIR) windows of about1.5-1.8 microns and about 2-2.5 microns may be attractive because theatmosphere transmits in these wavelength ranges. Although the sun can bea bright and stable light source, its illumination may be affected bythe time-of-day variations in the sun angle as well as weatherconditions. For example, the sun may be advantageously used forapplications such as hyper-spectral imaging only between about 9 am to 3pm, and it may be difficult to use the sun during cloudy days or duringinclement weather. In one embodiment, the hyper-spectral sensors measurethe reflected solar signal at hundreds (e.g., 100 to 200+) contiguousand narrow wavelength bands (e.g., bandwidth between 5 nm and 10 nm).Hyper-spectral images may provide spectral information to identify anddistinguish between spectrally similar materials, providing the abilityto make proper distinctions among materials with only subtle signaturedifferences. In the SWIR wavelength range, numerous gases, liquids andsolids have unique chemical signatures, particularly materialscomprising hydro-carbon bonds, O—H bonds, N—H bonds, etc. Therefore,spectroscopy in the SWIR may be attractive for stand-off or remotesensing of materials based on their chemical signature, which maycomplement other imaging information.

A SWIR super-continuum (SC) source may be able to replace at least inpart the sun as an illumination source for active remote sensing,spectroscopy, or hyper-spectral imaging. In one embodiment, reflectedlight spectroscopy may be implemented using the SWIR light source, wherethe spectral reflectance can be the ratio of reflected energy toincident energy as a function of wavelength. Reflectance varies withwavelength for most materials because energy at certain wavelengths maybe scattered or absorbed to different degrees. Using a SWIR light sourcemay permit 24/7 detection of solids, liquids, or gases based on theirchemical signatures. As an example, natural gas leak detection andexploration may require the detection of methane and ethane, whoseprimary constituents include hydro-carbons. In the SWIR, for instance,methane and ethane exhibit various overtone and combination bands forvibrational and rotational resonances of hydro-carbons. In oneembodiment, diffuse reflection spectroscopy or absorption spectroscopymay be used to detect the presence of natural gas. The detection systemmay include a gas filter correlation radiometer, in a particularembodiment. Also, one embodiment of the SWIR light source may be anall-fiber integrated SWIR SC source, which leverages the maturetechnologies from the telecommunications and fiber optics industry.Beyond natural gas, active remote sensing in the SWIR may also be usedto identify other materials such as vegetation, greenhouse gases orenvironmental pollutants, soils and rocks, plastics, illicit drugs,counterfeit drugs, firearms and explosives, paints, and various buildingmaterials.

In one embodiment, a measurement system comprises a light sourceconfigured to generate an output optical beam. The light sourcecomprises a plurality of semiconductor sources configured to generate aninput optical beam, a multiplexer configured to receive at least aportion of the input optical beam and to form an intermediate opticalbeam, and one or more fibers configured to receive at least a portion ofthe intermediate optical beam and to form the output optical beam. Atleast a portion of the one or more fibers comprises a fused silica fiberhaving a core diameter less than 400 microns. The output optical beamcomprises one or more optical wavelengths, at least a portion of whichare between 700 nanometers and 2500 nanometers, and has a bandwidth ofat least 10 nanometers. The measurement system also includes ameasurement apparatus configured to receive a received portion of theoutput optical beam and to deliver a delivered portion of the outputoptical beam to a sample, wherein the delivered portion of the outputoptical beam is configured to generate a spectroscopy output beam fromthe sample. A receiver is configured to receive at least a portion ofthe spectroscopy output beam having a bandwidth of at least 10nanometers and to process the at least a portion of the spectroscopyoutput beam to generate an output signal. The light source and thereceiver are remote from the sample, which comprises plastics or foodindustry goods, and the measurement system performs non-destructivequality control or constitutive analysis.

In one embodiment, a measurement system comprises a light sourceconfigured to generate an output optical beam. The light sourcecomprises a plurality of semiconductor sources configured to generate aninput optical beam, a multiplexer configured to receive at least aportion of the input optical beam and to form an intermediate opticalbeam, and one or more fibers configured to receive at least a portion ofthe intermediate optical beam and to form the output optical beam. Atleast a portion of the one or more fibers comprises a fused silica fiberhaving a core diameter less than 400 microns and the output optical beamcomprises one or more optical wavelengths, at least a portion of whichare between 700 nanometers and 2500 nanometers, with a bandwidth of atleast 10 nanometers. The system includes a measurement apparatusconfigured to receive a received portion of the output optical beam andto deliver a delivered portion of the output optical beam to a sample,wherein the delivered portion of the output optical beam is configuredto generate a spectroscopy output beam from the sample. A receiver isconfigured to receive at least a portion of the spectroscopy output beamhaving a bandwidth of at least 10 nanometers and to process the at leasta portion of the spectroscopy output beam to generate an output signal.The light source and the receiver are remote from the sample, whichcomprises plastics or food industry goods.

In one embodiment, a method of measuring comprises generating an outputoptical beam, comprising generating an input optical beam from aplurality of semiconductor sources, multiplexing at least a portion ofthe input optical beam and forming an intermediate optical beam, andguiding at least a portion of the intermediate optical beam using one ormore fibers including a fused silica fiber having a core diameter lessthan 400 microns. The method includes broadening a spectrum of at leasta portion of the intermediate optical beam to at least 10 nanometers toform the output optical beam with an output beam broadened spectrum,wherein the output optical beam comprises one or more opticalwavelengths between 700 nanometers and 2500 nanometers. The method alsoincludes receiving a received portion of the output optical beam anddelivering a delivered portion of the output optical beam to a samplecomprising plastics or food industry goods located remotely from thegenerated output optical beam, and generating a spectroscopy output beamhaving a bandwidth of at least 10 nanometers from the sample, whereinthe spectroscopy output beam comprises spectral features based on achemical composition of the sample. The method further includesreceiving at least a portion of the spectroscopy output beam, andprocessing the at least a portion of the spectroscopy output beam andgenerating an output signal.

In one embodiment, a measurement system comprises a light sourceconfigured to generate an output optical beam, the light sourcecomprising one or more semiconductor sources configured to generate aninput beam, one or more optical amplifiers configured to receive atleast a portion of the input beam and to deliver an intermediate beam toan output end of the one or more optical amplifiers, one or more opticalfibers configured to receive at least a portion of the intermediate beamand to deliver at least the portion of the intermediate beam to a distalend of the one or more optical fibers to form a first optical beam, anonlinear element configured to receive at least a portion of the firstoptical beam and to broaden a spectrum associated with the at least aportion of the first optical beam to at least 10 nm through a nonlineareffect in the nonlinear element to form the output optical beam with anoutput beam broadened spectrum, wherein at least a portion of the outputbeam broadened spectrum comprises a near-infrared wavelength betweenapproximately 700 nanometers and approximately 2500 nanometers, andwherein at least a portion of the one or more fibers is a fused silicafiber with a core diameter less than approximately 400 microns. Thesystem also includes a measurement apparatus configured to receive areceived portion of the output optical beam and to deliver a deliveredportion of the output optical beam to a sample, wherein the deliveredportion of the output optical beam is configured to generate aspectroscopy output beam from the sample, and a receiver configured toreceive at least a portion of the spectroscopy output beam having abandwidth of at least 10 nanometers and to process the at least aportion of the spectroscopy output beam to generate an output signal,wherein the light source and the receiver are remote from the sample,and wherein the sample comprises plastics or food industry goods.

In another embodiment, a measurement system comprises a light sourceconfigured to generate an output optical beam, the light sourcecomprising a plurality of semiconductor sources configured to generatean input optical beam, a multiplexer configured to receive at least aportion of the input optical beam and to form an intermediate opticalbeam, one or more fibers configured to receive at least a portion of theintermediate optical beam and to form the output optical beam, whereinat least a portion of the one or more fibers comprises a fused silicafiber having a core diameter less than approximately 400 microns,wherein the output optical beam comprises one or more opticalwavelengths, at least a portion of which are between approximately 700nanometers and approximately 2500 nanometers, and wherein the outputoptical beam has a bandwidth of at least 10 nanometers. The systemincludes a measurement apparatus configured to receive a receivedportion of the output optical beam and to deliver a delivered portion ofthe output optical beam to a sample, wherein the delivered portion ofthe output optical beam is configured to generate a spectroscopy outputbeam from the sample, and a receiver configured to receive at least aportion of the spectroscopy output beam having a bandwidth of at least10 nanometers and to process the at least a portion of the spectroscopyoutput beam to generate an output signal, wherein the light source andthe receiver are remote from the sample, and wherein the samplecomprises plastics or food industry goods.

In another embodiment, a method of measuring comprises generating anoutput optical beam comprising generating an input optical beam from aplurality of semiconductor sources, multiplexing at least a portion ofthe input optical beam and forming an intermediate optical beam, guidingat least a portion of the intermediate optical beam using one or morefibers including a fused silica fiber having a core diameter less than400 microns, and broadening a spectrum of at least a portion of theintermediate optical beam to at least 10 nanometers to form the outputoptical beam with an output beam broadened spectrum, wherein the outputoptical beam comprises one or more optical wavelengths between 700nanometers and 2500 nanometers. The method also includes receiving areceived portion of the output optical beam and delivering a deliveredportion of the output optical beam to a sample comprising plastics orfood industry goods located remotely from the generated output opticalbeam, generating a spectroscopy output beam having a bandwidth of atleast 10 nanometers from the sample, wherein the spectroscopy outputbeam comprises spectral features of hydrocarbons or organic compounds,receiving at least a portion of the spectroscopy output beam, andprocessing the at least a portion of the spectroscopy output beam andgenerating an output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and forfurther features and advantages thereof, reference is now made to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 illustrates wavelength bands for different chemical compoundsover the SWIR wavelength range of approximately 1400 nm to 2500 nm. Alsoindicated are whether the bands are overtone or combination bands.

FIGS. 2A-2B show the absorption spectra for methane and ethane,respectively.

FIG. 3 illustrates the reflectance spectra for some members of thealkane family plus paraffin.

FIG. 4A depicts that micro-seepages may result from the verticalmovement of hydro-carbons from their respective reservoirs to thesurface. It is assumed that the rock column, including the seal rock,comprises interconnected fractures or micro-fracture systems.

FIG. 4B illustrates that surface alterations may occur because leakinghydro-carbons set up near-surface oxidation and/or reduction zones thatfavor the development of a diverse array of chemical and mineralogicalchanges.

FIG. 5A shows the reflectance spectra for locations with natural gasfields (501) and locations without natural gas fields (502).

FIG. 5B illustrates spectra from field tests over regions with naturalgas, which show two spectral features: one near 1.725 microns andanother doublet between about 2.311 microns and 2.36 microns.

FIG. 6 shows the reflectance spectra of a sample of oil emulsion fromthe Gulf of Mexico 2010 oil spill (different thicknesses of oil).

FIG. 7 illustrates the reflectance spectra of some representativeminerals that may be major components of rocks and soils.

FIG. 8 shows the reflectance spectra of different types of greenvegetation compared with dry, yellowed grass.

FIG. 9 illustrates the atmospheric absorption and scattering ofgreenhouse gases at different wavelengths.

FIG. 10 overlays the reflectance for different building materials fromthe ASTER spectra library.

FIG. 11 shows the absorbance for two common plastics, polyethylene andpolystyrene.

FIG. 12 shows the experimental set-up for a reflection-spectroscopybased stand-off detection system.

FIG. 13 illustrates the chemical structure and molecular formula forvarious explosives, along with the absorbance spectra obtained using asuper-continuum source.

FIG. 14A shows the reflection spectra for gypsum, pine wood, ammoniumnitrate and urea.

FIG. 14B illustrates the reflection spectra for three commercialautomotive paints and military grade CARC paint (chemical agentresistant coating) (reflectance in this case are in arbitrary units).

FIG. 15 shows the mid-wave infrared and long-wave infrared absorptionspectra for various illicit drugs. It is expected that overtone andcombination bands should be evident in the SWIR and near-infraredwavelength bands.

FIG. 16A is a schematic diagram of the basic elements of an imagingspectrometer.

FIG. 16B illustrates one example of a typical imaging spectrometer usedin hyper-spectral imaging systems.

FIG. 17 shows one example of a gas-filter correlation radiometer, whichis a detection system that uses a sample of the gas of interest as aspectral filter for the gas.

FIG. 18 exemplifies a dual-beam experimental set-up that may be used tosubtract out (or at least minimize the adverse effects of) light sourcefluctuations.

FIG. 19 illustrates a block diagram or building blocks for constructinghigh power laser diode assemblies.

FIG. 20 shows a platform architecture for different wavelength rangesfor an all-fiber-integrated, high powered, super-continuum light source.

FIG. 21 illustrates one preferred embodiment for a short-wave infraredsuper-continuum light source.

FIG. 22 shows the output spectrum from the SWIR SC laser of FIG. 21 whenabout 10 m length of fiber for SC generation is used. This fiber is asingle-mode, non-dispersion shifted fiber that is optimized foroperation near 1550 nm.

FIG. 23 illustrates high power SWIR-SC lasers that may generate lightbetween approximately 1.4-1.8 microns (top) or approximately 2-2.5microns (bottom).

FIG. 24 illustrates a flowchart of a smart manufacturing process.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

As required, detailed embodiments of the present disclosure aredescribed herein; however, it is to be understood that the disclosedembodiments are merely exemplary of the disclosure that may be embodiedin various and alternative forms. The figures are not necessarily toscale; some features may be exaggerated or minimized to show details ofparticular components. Therefore, specific structural and functionaldetails disclosed herein are not to be interpreted as limiting, butmerely as a representative basis for teaching one skilled in the art tovariously employ the present disclosure.

One advantage of optical systems is that they can perform non-contact,stand-off or remote sensing distance spectroscopy of various materials.For remote sensing particularly, it may also be necessary to operate inatmospheric transmission windows. For example, two windows in the SWIRthat transmit through the atmosphere are approximately 1.4-1.8 micronsand 2-2.5 microns. In general, the near-infrared region of theelectromagnetic spectrum covers between approximately 0.7 microns (700nm) to about 2.5 microns (2500 nm). However, it may also be advantageousto use just the short-wave infrared between approximately 1.4 microns(1400 nm) and about 2.5 microns (2500 nm). One reason for preferring theSWIR over the entire NIR may be to operate in the so-called “eye safe”window, which corresponds to wavelengths longer than about 1400 nm.Therefore, for the remainder of the disclosure the SWIR will be used forillustrative purposes. However, it should be clear that the discussionthat follows could also apply to using the NIR wavelength range, orother wavelength bands.

In particular, wavelengths in the eye safe window may not transmit downto the retina of the eye, and therefore, these wavelengths may be lesslikely to create permanent eye damage from inadvertent exposure. Thenear-infrared wavelengths have the potential to be dangerous, becausethe eye cannot see the wavelengths (as it can in the visible), yet theycan penetrate and cause damage to the eye. Even if a practitioner is notlooking directly at the laser beam, the practitioner's eyes may receivestray light from a reflection or scattering from some surface. Hence, itcan always be a good practice to use eye protection when working aroundlasers. Since wavelengths longer than about 1400 nm are substantiallynot transmitted to the retina or substantially absorbed in the retina,this wavelength range is known as the eye safe window. For wavelengthslonger than 1400 nm, in general only the cornea of the eye may receiveor absorb the light radiation.

The SWIR wavelength range may be particularly valuable for identifyingmaterials based on their chemical composition because the wavelengthrange comprises overtones and combination bands for numerous chemicalbonds. As an example, FIG. 1 illustrates some of the wavelength bandsfor different chemical compositions. In 100 is plotted wavelength rangesin the SWIR (between 1400 and 2500 nm) for different chemical compoundsthat have vibrational or rotational resonances, along with whether thebands are overtone or combination bands. Numerous hydro-carbons arerepresented, along with oxygen-hydrogen and carbon-oxygen bonds. Thus,gases, liquids and solids that comprise these chemical compounds mayexhibit spectral features in the SWIR wavelength range. In a particularembodiment, the spectra of organic compounds may be dominated by the C—Hstretch. The C—H stretch fundamental occurs near 3.4 microns, the firstovertone is near 1.7 microns, and a combination band occurs near 2.3microns.

One embodiment of remote sensing that is used to identify and classifyvarious materials is so-called “hyper-spectral imaging.” Hyper-spectralsensors may collect information as a set of images, where each imagerepresents a range of wavelengths over a spectral band. Hyper-spectralimaging may deal with imaging narrow spectral bands over anapproximately continuous spectral range. As an example, inhyper-spectral imaging the sun may be used as the illumination source,and the daytime illumination may comprise direct solar illumination aswell as scattered solar (skylight), which is caused by the presence ofthe atmosphere. However, the sun illumination changes with time of day,clouds or inclement weather may block the sun light, and the sun lightis not accessible in the night time. Therefore, it would be advantageousto have a broadband light source covering the SWIR that may be used inplace of the sun to identify or classify materials in remote sensing orstand-off detection applications.

As used throughout this document, the term “couple” and or “coupled”refers to any direct or indirect communication between two or moreelements, whether or not those elements are physically connected to oneanother. As used throughout this disclosure, the term “spectroscopy”means that a tissue or sample is inspected by comparing differentfeatures, such as wavelength (or frequency), spatial location,transmission, absorption, reflectivity, scattering, refractive index, oropacity. In one embodiment, “spectroscopy” may mean that the wavelengthof the light source is varied, and the transmission, absorption orreflectivity of the tissue or sample is measured as a function ofwavelength. In another embodiment, “spectroscopy” may mean that thewavelength dependence of the transmission, absorption or reflectivity iscompared between different spatial locations on a tissue or sample. Asan illustration, the “spectroscopy” may be performed by varying thewavelength of the light source, or by using a broadband light source andanalyzing the signal using a spectrometer, wavemeter, or opticalspectrum analyzer.

As used throughout this document, the term “fiber laser” refers to alaser or oscillator that has as an output light or an optical beam,wherein at least a part of the laser comprises an optical fiber. Forinstance, the fiber in the “fiber laser” may comprise one of or acombination of a single mode fiber, a multi-mode fiber, a mid-infraredfiber, a photonic crystal fiber, a doped fiber, a gain fiber, or, moregenerally, an approximately cylindrically shaped waveguide orlight-pipe. In one embodiment, the gain fiber may be doped with rareearth material, such as ytterbium, erbium, and/or thulium. In anotherembodiment, the mid-infrared fiber may comprise one or a combination offluoride fiber, ZBLAN fiber, chalcogenide fiber, tellurite fiber, orgermanium doped fiber. In yet another embodiment, the single mode fibermay include standard single-mode fiber, dispersion shifted fiber,non-zero dispersion shifted fiber, high-nonlinearity fiber, and smallcore size fibers.

As used throughout this disclosure, the term “pump laser” refers to alaser or oscillator that has as an output light or an optical beam,wherein the output light or optical beam is coupled to a gain medium toexcite the gain medium, which in turn may amplify another input opticalsignal or beam. In one particular example, the gain medium may be adoped fiber, such as a fiber doped with ytterbium, erbium and/orthulium. In one embodiment, the “pump laser” may be a fiber laser, asolid state laser, a laser involving a nonlinear crystal, an opticalparametric oscillator, a semiconductor laser, or a plurality ofsemiconductor lasers that may be multiplexed together. In anotherembodiment, the “pump laser” may be coupled to the gain medium by usinga fiber coupler, a dichroic mirror, a multiplexer, a wavelength divisionmultiplexer, a grating, or a fused fiber coupler.

As used throughout this document, the term “super-continuum” and or“supercontinuum” and or “SC” refers to a broadband light beam or outputthat comprises a plurality of wavelengths. In a particular example, theplurality of wavelengths may be adjacent to one-another, so that thespectrum of the light beam or output appears as a continuous band whenmeasured with a spectrometer. In one embodiment, the broadband lightbeam may have a bandwidth of at least 10 nm. In another embodiment, the“super-continuum” may be generated through nonlinear opticalinteractions in a medium, such as an optical fiber or nonlinear crystal.For example, the “super-continuum” may be generated through one or acombination of nonlinear activities such as four-wave mixing, parametricamplification, the Raman effect, modulational instability, andself-phase modulation.

As used throughout this disclosure, the terms “optical light” and or“optical beam” and or “light beam” refer to photons or light transmittedto a particular location in space. The “optical light” and or “opticalbeam” and or “light beam” may be modulated or unmodulated, which alsomeans that they may or may not contain information. In one embodiment,the “optical light” and or “optical beam” and or “light beam” mayoriginate from a fiber, a fiber laser, a laser, a light emitting diode,a lamp, a pump laser, or a light source.

As used throughout this disclosure, the term “remote sensing” mayinclude the measuring of properties of an object from a distance,without physically sampling the object, for example by detection of theinteractions of the object with an electromagnetic field. In oneembodiment, the electromagnetic field may be in the optical wavelengthrange, including the infrared or SWIR. One particular form of remotesensing may be stand-off detection, which may range from non-contact upto hundreds of meters away, for example.

Remote Sensing of Natural Gas Leaks

Natural gas may be a hydro-carbon gas mixture comprising primarilymethane, with other hydro-carbons, carbon dioxide, nitrogen and hydrogensulfide. Natural gas is important because it is an important energysource to provide heating and electricity. Moreover, it may also be usedas fuel for vehicles and as a chemical feedstock in the manufacture ofplastics and other commercially important organic chemicals. Althoughmethane is the primary component of natural gas, to uniquely identifynatural gas through spectroscopy requires monitoring of both methane andethane. If only methane is used, then areas like cow pastures could bemistaken for natural gas fields or leaks. More specifically, the typicalcomposition of natural gas is as follows:

Component Range (mole %) Methane 87.0-96.0 Ethane 1.5-5.1 Propane0.1-1.5 Iso-butane 0.01-0.3  Normal-butane 0.01-0.3  Iso-pentaneTrace-0.14 Normal-pentane Trace-0.04 Hexanes plus Trace-0.06 Nitrogen0.7-5.6 Carbon dioxide 0.1-1.0 Oxygen 0.01-0.1  Hydrogen Trace-0.02

As one example of remote sensing of natural gas, a helicopter oraircraft may be flown at some elevation. The light source for remotesensing may direct the light beam toward the ground, and the diffusereflected light may then be measured using a detection system on theaircraft. Thus, the helicopter or aircraft may be sampling a column areabelow it for natural gas, or whatever the material of interest is. Inyet another embodiment, the column may sense aerosols of various sorts,as an example. Various kinds of SWIR light sources will be discussedlater in this disclosure. The detection system may comprise, in oneembodiment, a spectrometer followed by one or more detectors. In anotherembodiment, the detection system may be a dispersive element (examplesinclude prisms, gratings, or other wavelength separators) followed byone or more detectors or detector arrays. In yet another embodiment, thedetection system may comprise a gas-filter correlation radiometer. Theseare merely specific examples of the detection system, but combinationsof these or other detection systems may also be used and arecontemplated within the scope of this disclosure. Also, the use ofaircraft is one particular example of a remote sensing system, but othersystem configurations may also be used and are included in the scope ofthis disclosure. For example, the light source and detection system maybe placed in a fixed location, and for reflection the light source anddetectors may be close to one another, while for transmission the lightsource and detectors may be at different locations. In yet anotherembodiment, the system could be placed on a vehicle such as anautomobile or a truck, or the light source could be placed on onevehicle, while the detection system is on another vehicle. If the lightsource and detection system are compact and lightweight, they might evenbe carried by a person in the field, either in their hands or in abackpack.

Both methane and ethane are hydro-carbons with unique spectralsignatures. For example, ethane is C₂H₆, while methane is CH₄. Also,methane and ethane have infrared absorption bands near 1.6 microns, 2.4microns, 3.3 microns and 7 microns. It should be noted that theapproximately 7 micron lines cannot be observed generally due toatmospheric absorption. Although the fundamental lines near 3.3 micronsare stronger absorption features, the light sources and detectors in themid-infrared may be more difficult to implement. Hence, the focus hereis on observing the SWIR lines that fall in atmospheric transparencywindows.

FIG. 2 illustrates the absorption spectra for methane (FIG. 2A) andethane (FIG. 2B) (from http://vpl.astro.washington.edu/spectra). Thecurves 200 plot on a linear scale the absorption cross-section versuswavelength (in microns) for various methane lines. The curve 201 coversthe wavelength range between approximately 1.5-16 microns, while thecurves below provide blown-up views of different wavelength ranges (202for approximately 1.62-1.7 microns, 203 for approximately 1.7-1.84microns, 204 for approximately 2.15-2.45 microns, and 205 forapproximately 2.45-2.65 microns). The curves 202 and 203 fall withinabout the first SWIR atmospheric transmission window betweenapproximately 1.4-1.8 microns, while the curves 204 and 205 fall withinthe second SWIR atmospheric transmission window between approximately2-2.5 microns. As can be seen, there are numerous spectral features foridentifying methane in the SWIR. In addition, there are even strongerfeatures near 3.4-3.6 microns and around 7-8 microns, although theserequire different light sources and detection systems.

FIG. 2B illustrates the absorption spectra for ethane. The curves 250plot on a linear scale the absorption cross-section versus wavelength(in microns) for various ethane lines. The curve 251 covers thewavelength range between approximately 1.5-16 microns, while the curve252 expands the scale between about 1.6-3.2 microns. The features 253fall within about the first SWIR atmospheric transmission window betweenapproximately 1.4-1.8 microns, while the features 254 and 255 fallwithin the second SWIR atmospheric transmission window betweenapproximately 2-2.5 microns. There are distinct spectral features foridentifying ethane as well in the SWIR. In addition, there are evenstronger features near 3.4-3.6 microns and around 7 microns.

For detecting natural gas leaks, a SWIR light source and a detectionsystem could be used in transmission or reflection. The area surroundingthe source or natural gas pipeline may be surveyed, and the detectionsystem may monitor the methane and ethane concentration, or even thepresence of these two gases. The region may be scanned to cover an arealarger than the laser beam. Also, if a certain quantity of natural gasis detected, an alarm may be set-off to alert the operator or peoplenearby. This is just one example of the natural gas leak detection, butother configurations and techniques may be used and are intended to becovered by this disclosure.

Natural gas leak detection is one example where active remote sensing orhyper-spectral imaging can be used to detect hydro-carbons or organiccompounds. However, there are many other examples where the techniquemay be used to perform reflectance spectroscopy of organic compounds,and these are also intended to be covered by this disclosure. In oneparticular embodiment, alkanes may be detected, where alkanes arehydro-carbon molecules comprising single carbon-carbon bonds. Alkaneshave the general formula C_(n)H_(2n+2) and are open chain, aliphatic ornon-cyclic molecules. Below are examples of some of the alkanes, whichinclude methane and ethane, as well as more complicated compounds.

Formula Methane CH₄ Ethane C₂H₆ Propane C₃H₈ Butane C₄H₁₀ Pentane C₅H₁₂Hexane C₆H₁₄ Heptane C₇H₁₆ Octane C₈H₁₈ Nonane C₉H₂₀ Decane C₁₀H₂₂Paraffin C₂₀₊C₄₂₊ Polyethylene (C₂H₄)_(n) or (LDPE, HDPE) (CH₂CH₂)_(n)Polyvinylchloride (C₂H₃Cl)_(n) or (PVC) (CHClCH₂)_(n) Polypropylene(C₃H₅)_(n) or {CH(CH₃)CH₂}_(n) Polyethylene terephthalate C₁₀H₈O₄ or(PETE) {(CO₂)₂C₆H₄(CH₂)₂}_(n) Nylon (polyamide) C₁₂H₂₄O₄N₂ or{C₁₀H₂₂(CO₂)₂(NH)₂}_(n)

FIG. 3 illustrates the reflectance spectra 300 for some members of thealkane family plus paraffin. The vertical lines indicate positions ofconstant wavelength and are aligned with apparent absorptions in themethane spectrum at 1.19, 1.67, 2.32, 3.1, 4.23 and 4.99 microns. Thespectra ore offset to enable easier viewing, and the offsets are of thefollowing amounts: 301 methane 4.1; 302 ethane 3.6; 303 propane 3.3; 304butane 2.8; 305 pentane 2.3; 306 hexane 2.0; 307 heptane 1.5; 308 octane1.2; 309 nonane 0.85; 310 decane 0.4; and 311 paraffin 0.05. Thereflectance of alkanes in the near-infrared may be dominated byabsorptions due to combinations and overtones of bands at longerwavelengths. Although this wavelength range is mostly unexplored byorganic spectroscopists, the near-infrared may be valuable forterrestrial and planetary remote sensing studies. Alkanes may have thefundamental absorptions due to a variety of C—H stretches betweenapproximately 3.3-3.5 microns. The first overtone may be a relativelydeep triplet near 1.7 microns. This triplet appears in most of theseries, but the exact wavelength position may move. Another absorptionband may be present near 1.2 microns, and this is likely the secondovertone of the C—H stretch. The third C—H stretch overtone is near 0.9microns. There is yet another near-infrared feature near 1.396 microns,which may correspond to the combinations of the first overtone of theC—H stretch with each of the two C—H band positions at approximately1.35 microns and 1.37 microns. Moreover, there may be complexabsorptions between 2.2-2.5 microns. For example, there may be a numberof narrow individual absorption bands atop an overall absorption suiteabout 0.3 microns wide. A few absorption lines retain their location formost of the series 300, notably the 2.311 micron and 2.355 micronabsorptions. This wavelength window may have multiple combinations andovertones, including contributions from the C—H stretch, CH₃ asymmetricbend combination, and C—H stretch/CH₃ symmetric bend combination.

Remote Sensing for Natural Gas Exploration

In addition to remote sensing to detect natural gas leaks, the same orsimilar system could also be used to explore for natural gas fields,whether under land or under water. Whereas a natural gas leak from apipeline or building may be above the ground or only a few meters belowthe ground, natural gas exploration may occur for gas and oil that aremuch further below the ground, or under the water in a bay, lake, sea orocean. For example, the exploration for natural gas and oil may beperformed by determining the reflectance spectra of surface anomalies.The surface manifestations of oil and gas reservoirs may be used to mapthe petroleum potential of an area, particularly related to the seepageof oil and gas to the surface along faults or imperfect reservoir seals.The visible products of such seepage (e.g., oil and tar deposits) aregenerally referred to as macro-seeps, whereas the invisible gaseousproducts may be referred to as micro-seeps.

As illustrated by 400 in FIG. 4, micro-seepages may result from thevertical movement of hydrocarbons 401 from their respective reservoirsto the surface. These hydrocarbon micro-seepages involve buoyant,relatively rapid, vertical ascent of ultra-small bubbles of lighthydrocarbons (primarily methane through the butanes) through a networkof interconnected, groundwater-filled joints and bedding planes (401).One of the assumptions required for micro-seepage to occur is that arock column, including the seal rock, comprises extensive interconnectedfractures or micro-fracture systems.

Direct detection methods may involve measurements of hydrocarbons,either in the form of oil accumulations or concentrations of escapingvapors, such as methane through butane. In addition, there are alsoindirect methods that may involve the measurement of secondaryalternations that arise from the seepage of the hydrocarbons. Forinstance, hydrocarbon-induced alterations may include microbialanomalies, mineralogical changes, bleaching of red beds, clay mineralalterations, and electrochemical changes. These alterations occurbecause leaking hydrocarbons set up near-surface oxidation and/orreduction zones that favor the development of a diverse array ofchemical and mineralogical changes, c.f. 402 in FIG. 4. Such alterations402 may be distinct from adjacent rocks and, thus, may in some instancebe detectable by various remote sensing techniques.

The diagnostic spectral features of methane and crude oil may comprisefour distinct hydrocarbon absorption bands. For example, two bands near1.18 microns and 1.38 microns may be narrow and sharply defined,although they may also be fairly weak. The other two spectral featuresmay be near 1.68-1.72 microns and 2.3-2.45 microns; these bands may bebroader, but they are also stronger than the previous two bands. Thebands near 1.7 microns and 2.3 microns are spectral overtones orcombinations of C—H vibrational modes. Moreover, hydrocarbon inducedalterations associated with indirect detection may express themselves ina variety of spectral changes, such as mineralogical changes (calciumcarbonate mineralization, near 2.35 microns), bleaching of red beds(near 1 micron), and clay minerals alterations (near 2.2 microns), amongother changes.

Various field tests have been conducted that verify the spectralsignatures associated with natural gas fields, either land-based orwater-based (e.g., in bays). In one example shown in FIG. 5A, thereflectance spectra 500 was collected for different locations betweenapproximately 2 microns and 2.4 microns. In 501 the reflectance isplotted versus wavelength for locations with gas fields, while in 502the reflectance is plotted for locations without gas fields. Themacroscopic features of the reflectance spectra of surface soils showtwo broad absorption bands near 2.2 microns and 2.33 microns withcomplex shapes. The slightly positive slope in the region of 2.3-2.4microns with natural gas suggests that hydrocarbons are overriding thespectral signature of clays in this region.

In yet another embodiment, field tests were conducted over a widerspectra range from approximately 0.5 microns to 2.5 microns (FIG. 5B).As the curve 550 illustrates, two absorption features are found for thehydrocarbon spectral reflectance curve: one near 1.725 microns 551 and adouble absorption at approximately 2.311-2.36 microns 552. Thus, inthese two field trial examples, oil-gas reservoir areas wereidentifiable using feature bands of 1650-1750 nm and 2000-2400 nm. Inaddition, the remote sensing method may be used for off-shore oil andgas exploration and marine pollution investigation, to name just a fewexamples.

Other Uses of Active Remote Sensing or Hyperspectral Imaging

Active and/or hyper-spectral remote sensing may be used in a wide arrayof applications. Although originally developed for mining and geology(the ability of spectral imaging to identify various minerals may beideal for the mining and oil industries, where it can be used to lookfor ore and oil), hyper-spectral remote sensing has spread to fields asdiverse as ecology and surveillance. The table below illustrates some ofthe applications that can benefit from hyper-spectral remote sensing.

Atmosphere water vapor, cloud properties, aerosols Ecology chlorophyll,leaf water, cellulose, pigments, lignin Geology mineral and soil typesCoastal Waters chlorophyll, phytoplankton, dissolved organic materials,suspended sediments Snow/Ice snow cover fraction, grainsize, meltingBiomass Burning subpixel temperatures, smoke Commercial mineral (oil)exploration, agriculture and forest production

In one embodiment, near-infrared imaging spectroscopy data may be usedto create qualitative images of thick oil or oil spills on water. Thismay provide a rapid remote sensing method to map the locations of thickparts of an oil spill. While color imagery may show locations of thickoil, it is difficult to assess relative thickness or volume with justcolor imagery. As an example, FIG. 6 illustrates the reflectance spectra600 of a sample of oil emulsion from the Gulf of Mexico 2010 oil spill.Curve 601 is a 4 mm thickness of oil, while curve 602 is a 0.5 mmthickness. Whereas the data in the visible hardly changes with oilthickness, in the near-infrared the change in reflectance spectra ismuch more dependent on the oil thickness. The data shows, for example,the C—H features near 1.2 microns 603, 1.73 microns 604, and 2.3 microns605. Thus, in the infrared wavelengths, both the reflectance levels andabsorption features due to organic compounds may vary in strength withoil thickness.

Remote sensing may also be used for geology and mineralogy mapping orinspection. FIG. 7 shows the reflectance spectra 700 for somerepresentative minerals that are major components of rocks and soils. Ininorganic materials such as minerals, chemical composition andcrystalline structure may control the shape of the spectral curve andthe locations of absorption bands. Wavelength-specific absorption mayarise from particular chemical elements or ions and the geometry ofchemical bonds between elements, which is related to the crystalstructure. In hematite 701, the strong absorption in the visible may becaused by ferric iron. In calcite 705, the carbonate ion may beresponsible for the series of absorption bands between 1.8 and 2.4microns. Kaolinite 704 and montmorillonite 702 are clay minerals commonin soils. The strong absorption near 1.4 microns in both spectra, alongwith a weak 1.9 micron band in kaolinite arise from the hydroxide ions,while the stronger 1.9 micron band in montmorillonite may be caused bybound water molecules in the hydrous clay. In contrast to these spectra,orthoclase feldspar 703, a dominant mineral in granite, shows verylittle absorption features in the visible or infrared.

Remote sensing or hyper-spectral imaging may also be used foragriculture as well as vegetation monitoring. For example,hyper-spectral data may be used to detect the chemical composition ofplants, which can be used to detect the nutrient and water status ofcrops. FIG. 8 illustrates the reflectance spectra 800 of different typesof green vegetation compared with dry, yellowed grass. In the visiblespectra, the shape may be determined by absorption effects fromchlorophyll and other leaf pigments. The reflectance rises rapidlyacross the boundary between red and infrared wavelengths, which may bedue to interactions with the internal cellular structure of leaves. Leafstructure may vary significantly between plant species, as well as fromplant stress. Beyond 1.3 microns the reflectance decreases withincreasing wavelength, except for two water absorption bands near 1.4microns and 1.9 microns. Illustrated in FIG. 8 are the reflectance forgreen grass 801, walnut tree canopy 802, fir tree 803 and senescent 804,which is dry, yellowed grass.

Active remote sensing may also be used to measure or monitor gases inthe earth's atmosphere, including greenhouse gases, environmentalpollutants and aerosols. For instance, greenhouse gases are those thatcan absorb and emit infrared radiation: In order, the most abundantgreenhouse gasses in the Earth's atmosphere are: water vapor (H₂O),carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃).FIG. 9 shows the atmospheric absorption and scattering of greenhousegases 900 at different wavelengths. Included in this figure are thetotal absorption and scattering 901, along with the breakdown by majorcomponents: water vapor 902, carbon dioxide 903, oxygen and ozone 904,methane 905, and nitrous oxide 906. Also shown is the Rayleighscattering 907 through the atmosphere, which dominates at shorterwavelengths, particularly wavelengths shorter than about 1 micron. Inone embodiment, environmental concerns of climate change have led to theneed to monitor the level of carbon dioxide in the atmosphere, and thismay be achieved, for example, by performing spectroscopy in the vicinityof 1.6 microns and 2 microns.

In yet another embodiment, different building materials may beidentified and distinguished from surrounding vegetation and forestry.FIG. 10 overlays different reflectance data 1000 for samples catalogedin the ASTER spectra library (http://speclib.jpl.nasa.gov). This libraryhas been made available by NASA as part of the Advanced SpaceborneThermal Emission and Reflection Radiometer, ASTER, imaginginstrumentation program. Included in this and other libraries arereflection spectra of natural and man-made materials, includingminerals, rocks, soils, water and snow. In FIG. 10 several spectra areincluded over the SWIR atmospheric transmission bands, and the waterabsorption between approximately 1.8 and 2 microns has been blocked out(features in there are either due to water or would be masked by theatmospheric moisture). Included in the graph are the spectra for silvermetallic paint 1001, light brown loamy sand 1002, constructionconcrete-1 1003, construction concrete-cement 1004, gypsum 1005,asphaltic concrete 1006, construction concrete-bridges 1007, grass 1008and conifer trees 1009. As an example, active remote sensing can be usedto distinguish different concrete structures, including roadways,buildings, and reinforced structures such as bridges. Also, buildingmaterials such as gypsum, painted structures, plywood, and concrete ofvarious sorts, may be distinguished from plant life, soil and trees.Thus, beyond three dimensional imaging, this can add a fourthdimension—namely, identification of objects based on their chemicalsignature.

In a further embodiment, remote sensing or hyper-spectral imaging mightbe used for process control in a factory or manufacturing setting,particularly when the measurements are to be made at some stand-off orremote distance. As an example, plastics show distinct signatures in theSWIR, and process control may be used for monitoring the manufacture ofplastics. Alternately, SWIR light could be used to see through plastics,since the signature for plastics can be subtracted off and there arelarge wavelength windows where the plastics are transparent. FIG. 11illustrates the absorbance 1100 for two common plastics: polyethylene1101 and polystyrene 1102. Because of the hydro-carbon bonds, there areabsorption features near 1.7 microns and 2.2-2.5 microns (c.f.,discussion on alkanes). In general, the absorption bands in the nearinfrared are due to overtones and combination bands for variousfunctional group vibrations, including signals from C—H, O—H, C═O, N—H,—COOH, and aromatic C—H groups. It may be difficult to assign anabsorption band to a specific functional group due to overlapping ofseveral combinations and overtones. However, with advancements incomputational power and chemometrics or multivariate analysis methods,complex systems may be better analyzed. In one embodiment, usingsoftware analysis tools the absorption spectrum may be converted to itssecond derivative equivalent. The spectral differences may permit afast, accurate, non-destructive and reliable identification ofmaterials. Although particular derivatives are discussed, othermathematical manipulations may be used in the analysis, and these othertechniques are also intended to be covered by this disclosure.

In another specific embodiment, experiments have been performed forstand-off detection of solid targets with diffuse reflectionspectroscopy using a fiber-based super-continuum source (furtherdescribed herein). In particular, the diffuse reflection spectrum ofsolid samples such as explosives (TNT, RDX, PETN), fertilizers (ammoniumnitrate, urea), and paints (automotive and military grade) have beenmeasured at stand-off distances of 5 m. Although the measurements weredone at 5 m, calculations show that the distance could be anywhere froma few meters to over 150 m. These are specific samples that have beentested, but more generally other materials (particularly comprisinghydro-carbons) could also be tested and identified using similarmethods. The experimental set-up 1200 for thereflection-spectroscopy-based stand-off detection system is shown inFIG. 12, while details of the SC source 1201 are described later in thisdisclosure (c.f. FIGS. 20,21, and 23). First, the diverging SC output iscollimated to a 1 cm diameter beam using a 25 mm focal length, 90degrees off-axis, gold coated, parabolic mirror 1202. To reduce theeffects of chromatic aberration, refractive optics are avoided in thesetup. All focusing and collimation is done using metallic mirrors thathave almost constant reflectivity and focal length over the entire SCoutput spectrum. The sample 1204 is kept at a distance of 5 m from thecollimating mirror 1202, which corresponds to a total round trip pathlength of 10 m before reaching the collection optics 1205. A 12 cmdiameter silver coated concave mirror 1205 with a 75 cm focal length iskept 20 cm to the side of the collimation mirror 1202. The mirror 1205is used to collect a fraction of the diffusely reflected light from thesample, and focus it into the input slit of a monochromator 1206. Thus,the beam is incident normally on the sample 1204, but detected at areflection angle of tan⁻¹(0.2/5) or about 2.3 degrees. Appropriate longwavelength pass filters mounted in a motorized rotating filter wheel areplaced in the beam path before the input slit 1206 to avoid contributionfrom higher wavelength orders from the grating (300 grooves/mm, 2 μmblaze). The output slit width is set to 2 mm corresponding to a spectralresolution of 10.8 nm, and the light is detected by a 2 mm×2 mm liquidnitrogen cooled (77K) indium antimonide (InSb) detector 1207. Thedetected output is amplified using a trans-impedance pre-amplifier 1207with a gain of about 105V/A and connected to a lock-in amplifier 1208setup for high sensitivity detection. The chopper frequency is 400 Hz,and the lock-in time constant is set to 100 ms corresponding to a noisebandwidth of about 1 Hz. These are exemplary elements and parametervalues, but other or different optical elements may be used consistentwith this disclosure.

Three sets of solid samples are chosen to demonstrate the stand-offdiffuse reflection spectra measurement in the laboratory. The first setcomprises ‘Non-hazardous Explosives for Security Training and Testing’(NESTT) manufactured by the XM Division of VanAken International. Thesesamples contain small amounts of explosives deposited on an inert fusedsilica powder substrate. The experiments are conduced with the followingsamples—trinitrotoluene (TNT), research department explosive (RDX),Pentaerythritol tetranitrate (PETN), and potassium nitrate. The TNT, RDXand potassium nitrate NESTT samples have 8% (by weight) explosives,while the PETN sample has 4%.

The second sample set consists of ammonium nitrate, urea, gypsum, andpinewood. Ammonium nitrate and urea are common fertilizers, but are alsooften used as explosives. These samples are ground to a fine powder in amortar and pestle, and filled to a depth of about 5 mm in a shallowglass container. We also measure the reflection spectrum of a 10 cmdiameter×0.5 cm thick Gypsum (CaSO₄.2H₂O) disk and a 5 cm×5 cm×0.5 mpiece of pine wood, since these samples are relevant for the remotesensing community (minerals and vegetation).

The final set of samples is selected to distinguish between commercialautomotive and military vehicle paints based on their reflectionsignatures. Red, black, and green acrylic based spray paints areobtained from an auto supply store and sprayed 3 coats on differentareas of a sanded Aluminum block to make the automotive paint samples.The sample of the military paint consisted of an Aluminum block coatedwith a chemical agent resistant coating (CARC) green paint.

The chemical structure and molecular formula of the 4 NESTT samples areshown in FIG. 13 (1301, 1302, 1303, 1304), while the absorbance spectraobtained using the SC source are shown below in the same FIG. 1305,1306, 1307, 1308). For each sample, the positions of thestrongest/unique peaks have been labeled for clarity. TNT 1301, 1305belongs to a class of compounds known as nitro-aromatics, in which thecarbon directly attached to the nitro (NO₂) group is part of an aromaticring. The strongest peaks in the spectrum observed at 3230 nm and 3270nm are due to the fundamental C—H stretching vibrations in the aromaticring. There are also features between 2200-2600 nm, which may arise fromthe combination between the C—H stretch and C—H bend vibrations. RDX1302, 1306 belongs to the nitramines class containing the N—NO₂ bond andalso has multiple features in the 3200-3500 nm band due to the C—Hstretch vibrations. This spectrum also contains the C—H combinationbands from 2200-2600 nm. PETN 1303, 1307 is classified as a nitrateester containing the C—O—NO₂ bond, and its reflection spectrum ischaracterized by a triplet of peaks at 3310 nm, 3350 nm and 3440 nm dueto the C—H stretch vibration from the aliphatic groups. The C—Hcombination band is also present from 2200-2600 nm. Potassium nitrate1304, 1308 being an inorganic compound does not contain any absorptionfeatures due to the C—H bond present in the other three samples.Instead, the unique spectral feature for this sample is a pair of peaksat 3590 nm and 3650 nm, which arise due to the first overtone of theasymmetric N—O stretching vibration of the nitrate ion (NO₃ ⁻).

FIG. 14A illustrates the reflection spectra 1400 for gypsum 1401,pinewood 1402, ammonium nitrate 1403 and urea 1404. The predominantspectral features in the gypsum 1401 (CaSO₄.2H₂O) reflectance occur dueto the fundamental as well as combination bands of the water moleculenear 1450 nm, 1750 nm, 1940 nm and 2860 nm. In addition, small dips inthe spectrum at 2220, 2260 and 2480 nm which arise due to the firstovertone of the S—O bending vibration. Moreover, the valley at 3970 nmoccurs due to the first overtone of the —O—S—O stretching vibration ofthe sulfate (SO₄ ²⁻) ion. The pine wood spectrum 1402 comprises of bandsdue to its main constituents—cellulose, lignin and water. The valleys at1450 nm, 1920 nm and 2860 nm are attributed to water. The dip at 2100 nmis due to the first overtone of the C—O asymmetric stretch, the one at2270 nm due to the combination band of O—H and C—H, and the one at 2490nm due to combination band of C—H and C—O. Finally, the broad featurearound 3450 nm is due to the C—H stretching vibration. The ammoniumnitrate (NH₄NO₃) spectrum 1403 has three prominent features in thenear-IR region. The dip at 1270 nm is due to the combination of N—Hstretching and N—H bending vibrations, while the dip at 1570 nm is dueto the first overtone of N—H stretch. The doublet at 2050 nm and 2140 nmis possibly due to the second overtone of the N—H bending vibrations,while the fundamental N—H stretch appears as a broad feature around 3000nm. Urea (NH₂)₂CO 1404 has two amide (—NH₂) groups joined by a carbonyl(C═O) functional group. The absorption line at 1490 nm occurs due to thethird overtone of the C═O stretching vibration while the line at 1990 nmis due to the second overtone of the same.

FIG. 14B shows the reflection spectra 1450 for three commercialautomotive paints 1451, 1452, 1453 and military grade CARC (chemicalagent resistant coating) paint 1454. The paints consist of a complexmixture of many different chemicals, and, hence, it is very difficult toidentify individual absorption lines. Since all four paints contain avariety of organic compounds, features are observed between 3200-3500 nmfrom the C—H stretch and from 2200-2600 nm due to the C—H stretch andC—H bond combination band. However, the primary difference between theautomotive 1451, 1452, 1453 and CARC paint 1454 is the presence of astrong dip between 1200-1850 nm in the latter, which might be attributedto the absorption from Cobalt chromite—a green pigment found inCARC-green.

Thus, FIGS. 13 and 14 show that various materials, including explosives,fertilizers, vegetation, and paints have features in the near-infraredand SWIR that can be used to identify the various samples. Althoughstronger features are found in the mid-infrared, the near-infrared maybe easier to measure due to higher quality detection systems, moremature fiber optics and light sources, and transmission throughatmospheric transmission windows. Because of these distinct spectralsignatures, these materials could also be detected using active remotesensing or hyper-spectral imaging, as described in this disclosure.These are just particular samples that have been tested at stand-offdistances, but other materials and samples may also be identified usingthe SWIR remote sensing or hyper-spectral imaging methods, and thesesamples are also intended to be covered within this disclosure. As justanother example, illicit drugs may be detectable using remote sensing orhyper-spectral imaging. FIG. 15 shows the mid-wave infrared andlong-wave infrared absorption spectra 1500 for various illicit drugs.The absorbance for cocaine 1501, methamphetamine 1502, MDMA (ecstasy)1503, and heroin 1504 are plotted versus wavelength from approximately2.5-20 microns. Although the fundamental resonances for these drugs maylie in the longer wavelength regions, there are corresponding overtonesand combination bands in the SWIR and near-infrared wavelength range.Therefore, the active remote sensing or hyper-spectral imagingtechniques described herein may also be applicable to detecting illicitdrugs from aircraft, vehicles, or hand held devices.

Detection Systems

As discussed earlier, the active remote sensing system or hyper-spectralimaging system may be on an airborne platform, mounted on a vehicle, astationary transmission or reflection set-up, or even held by a humanfor a compact system. For such a system, there are fundamentally twohardware parts: the transmitter or light source and the detectionsystem. Between the two, perhaps in a transmission or reflectionsetting, may be the sample being tested or measured. Moreover, theoutput from the detection system may go to a computational system,comprising computers or other processing equipment. The output from thecomputational system may be displayed graphically as well as withnumerical tables and perhaps an identification of the materialcomposition. These are just some of the parts of the systems, but otherelements may be added or be eliminated, and these modifiedconfigurations are also intended to be covered by this disclosure.

By use of an active illuminator, a number of advantages may be achieved.First, the variations due to sunlight and time-of-day may be factoredout. The effects of the weather, such as clouds and rain, might also bereduced. Also, higher signal-to-noise ratios may be achieved. Forexample, one way to improve the signal-to-noise ratio would be to usemodulation and lock-in techniques. In one embodiment, the light sourcemay be modulated, and then the detection system would be synchronizedwith the light source. In a particular embodiment, the techniques fromlock-in detection may be used, where narrow band filtering around themodulation frequency may be used to reject noise outside the modulationfrequency. In an alternate embodiment, change detection schemes may beused, where the detection system captures the signal with the lightsource on and with the light source off. Again, for this system thelight source may be modulated. Then, the signal with and without thelight source is differenced. This may enable the sun light changes to besubtracted out. In addition, change detection may help to identifyobjects that change in the field of view. In the following someexemplary detection systems are described.

In one embodiment, a SWIR camera or infrared camera system may be usedto capture the images. The camera may include one or more lenses on theinput, which may be adjustable. The focal plane assemblies may be madefrom mercury cadmium telluride material (HgCdTe), and the detectors mayalso include thermo-electric coolers. Alternately, the image sensors maybe made from indium gallium arsenide (InGaAs), and CMOS transistors maybe connected to each pixel of the InGaAs photodiode array. The cameramay interface wirelessly or with a cable (e.g., USB, Ethernet cable, orfiber optics cable) to a computer or tablet or smart phone, where theimages may be captured and processed. These are a few examples ofinfrared cameras, but other SWIR or infrared cameras may be used and areintended to be covered by this disclosure.

In another embodiment, an imaging spectrometer may be used to detect thelight received from the sample. For example, FIG. 16A shows a schematicdiagram 1600 of the basic elements of an imaging spectrometer. The inputlight 1601 from the sample may first be directed by a scanning mirrorand/or other optics 1602. An optical dispersing element 1603, such as agrating or prism, in the spectrometer may split the light into manynarrow, adjacent wavelength bands, which may then be passed throughimaging optics 1604 onto one or more detectors or detector arrays 1605.Some sensors may use multiple detector arrays to measure hundreds ofnarrow wavelength bands.

An example of a typical imaging spectrometer 1650 used in hyper-spectralimaging systems is illustrated in FIG. 16B. In this particularembodiment, the input light may be directed first by a tunable mirror1651. A front lens 1652 may be placed before the entrance slit 1653 andthe collector lens 1654. In this embodiment, the dispersing element is aholographic grating with a prism 1655, which separates the differentwavelength bands. Then, a camera lens 1656 may be used to image thewavelengths onto a detector or camera 1657.

FIGS. 16A and 16B provide particular examples, but some of the elementsmay not be used, or other elements may be added, and these embodimentsare also intended to be covered by this disclosure. For instance, ascanning spectrometer may be used before the detector, where a gratingor dispersive element is scanned to vary the wavelength being measuredby the detector. In yet another embodiment, filters may be used beforeone or more detectors to select the wavelengths or wavelength bands tobe measured. This may be particularly useful if only a few bands orwavelengths are to be measured. The filters may be dielectric filters,Fabry-Perot filters, absorption or reflection filters, fiber gratings,or any other wavelength selective filter. In an alternate embodiment, awavelength division multiplexer, WDM, may be used followed by one ormore detectors or detector arrays. One example of a planar wavelengthdivision multiplexer may be a waveguide grating router or an arrayedwaveguide grating. The WDM may be fiber coupled, and detectors may beplaced directly at the output or the detectors may be coupled throughfibers to the WDM. Some of these components may also be combined withthe configurations in FIGS. 16A and 16B.

While the above detection systems could be categorized as single pathdetection systems, it may be advantageous in some cases to usemulti-path detection systems. In one embodiment, when the aim is tomeasure particular gases or material (rather than identify out of alibrary of materials), it may advantageous to use gas-filter correlationradiometry (GFCR), such as 1700 in FIG. 17. A GFCR is a detection systemthat uses a sample of the gas of interest as a spectral filter for thegas. As shown in FIG. 17, the incoming radiation 1701 may first bepassed through a narrow band pass filter 1702. The beam may then besplit by a beam splitter 1703 along two paths; one path comprising a gascell filled with the gas of interest 1704 (known as the correlationcell) and the other path comprising no gas 1705. The light from eachpath may then be measured using two detectors 1706, 1707, and thesignals may then be analyzed 1708. The difference in the transmissionalong the two paths may correspond primarily to the absorption of thegas along the correlation cell path. This GFCR configuration may beadvantageous, for example, in the detection of natural gas. Since thegoal is to measure methane and ethane, the correlation cells may containthese gases, either in combination or separately. Although a particularconfiguration for the GFCR has been described, variations of thisconfiguration as well as addition of other components may also be usedand are intended to be covered by this disclosure. For example,collection optics and lenses may be used with this configuration, andvarious modulation techniques may also be used to increase the signal tonoise ratio.

In yet another example of multi-beam detection systems, a dual-beamset-up 1800 such as in FIG. 18 may be used to subtract out (or at leastminimize the adverse effects of) light source fluctuations. In oneembodiment, the output from an SC source 1801 may be collimated using acalcium fluoride (CaF₂)lens 1802 and then focused into the entrance slitof the monochromator 1803. At the exit slit, light at the selectedwavelength is collimated again and may be passed through a polarizer1804 before being incident on a calcium fluoride beam splitter 1805.After passing through the beam splitter 1805, the light is split into asample 1806 and reference 1807 arm to enable ratiometric detection thatmay cancel out effects of intensity fluctuations in the SC source 1801.The light in the sample arm 1806 passes through the sample of interestand is then focused onto a HgCdTe detector 1808 connected to a pre-amp.A chopper 1802 and lock-in amplifier 1810 setup enable low noisedetection of the sample arm signal. The light in the reference arm 1807passes through an empty container (cuvette, gas cell etc.) of the samekind as used in the sample arm. A substantially identical detector 1809,pre-amp and lock-in amplifier 1810 is used for detection of thereference arm signal. The signal may then be analyzed using a computersystem 1811. This is one particular example of a method to removefluctuations from the light source, but other components may be addedand other configurations may be used, and these are also intended to becovered by this disclosure.

Although particular examples of detection systems have been described,combinations of these systems or other systems may also be used, andthese are also within the scope of this disclosure. As one example,environmental fluctuations (such as turbulence or winds) may lead tofluctuations in the beam for active remote sensing or hyper-spectralimaging. A configuration such as illustrated in the representativeembodiment of FIG. 18 may be able to remove the effect of environmentalfluctuations. Yet another technique may be to “wobble” the light beamafter the light source using a vibrating mirror. The motion may lead tothe beam moving enough to wash out spatial fluctuations within the beamwaist at the sample or detection system. If the vibrating mirror isscanned faster than the integration time of the detectors, then thespatial fluctuations in the beam may be integrated out. Alternately,some sort of synchronous detection system may be used, where thedetection is synchronized to the vibrating frequency.

Light Sources for SWIR and Near Infrared

There are a number of light sources that may be used in the nearinfrared. To be more specific, the discussion below will consider lightsources operating in the short wave infrared (SWIR), which may cover thewavelength range of approximately 1400 nm to 2500 nm. Other wavelengthranges may also be used for the applications described in thisdisclosure, so the discussion below is merely provided as exemplarytypes of light sources. The SWIR wavelength range may be valuable for anumber of reasons. The SWIR corresponds to a transmission window throughwater and the atmosphere. Also, the so-called “eye-safe” wavelengths arewavelengths longer than approximately 1400 nm.

Different light sources may be selected for the SWIR based on the needsof the application. Some of the features for selecting a particularlight source include power or intensity, wavelength range or bandwidth,spatial or temporal coherence, spatial beam quality for focusing ortransmission over long distance, and pulse width or pulse repetitionrate. Depending on the application, lamps, light emitting diodes (LEDs),laser diodes (LD's), tunable LD's, super-luminescent laser diodes(SLDs), fiber lasers or super-continuum sources (SC) may beadvantageously used. Also, different fibers may be used for transportingthe light, such as fused silica fibers, plastic fibers, mid-infraredfibers (e.g., tellurite, chalcogenides, fluorides, ZBLAN, etc), or ahybrid of these fibers.

Lamps may be used if low power or intensity of light is required in theSWIR, and if an incoherent beam is suitable. In one embodiment, in theSWIR an incandescent lamp that can be used is based on tungsten andhalogen, which have an emission wavelength between approximately 500 nmto 2500 nm. For low intensity applications, it may also be possible touse thermal sources, where the SWIR radiation is based on the black bodyradiation from the hot object. Although the thermal and lamp basedsources are broadband and have low intensity fluctuations, it may bedifficult to achieve a high signal-to-noise ratio due to the low powerlevels. Also, the lamp based sources tend to be energy inefficient.

In another embodiment, LED's can be used that have a higher power levelin the SWIR wavelength range. LED's also produce an incoherent beam, butthe power level can be higher than a lamp and with higher energyefficiency. Also, the LED output may more easily be modulated, and theLED provides the option of continuous wave or pulsed mode of operation.LED's are solid state components that emit a wavelength band that is ofmoderate width, typically between about 20 nm to 40 nm. There are alsoso-called super-luminescent LEDs that may even emit over a much widerwavelength range. In another embodiment, a wide band light source may beconstructed by combining different LEDs that emit in at differentwavelength bands, some of which could preferably overlap in spectrum.One advantage of LEDs as well as other solid state components is thecompact size that they may be packaged into.

In yet another embodiment, various types of laser diodes may be used inthe SWIR wavelength range. Just as LEDs may be higher in power butnarrower in wavelength emission than lamps and thermal sources, the LDsmay be yet higher in power but yet narrower in wavelength emission thanLEDs. Different kinds of LDs may be used, including Fabry-Perot LDs,distributed feedback (DFB) LDs, distributed Bragg reflector (DBR) LDs.Since the LDs have relatively narrow wavelength range (typically under10 nm), in a preferred embodiment a plurality of LDs may be used thatare at different wavelengths in the SWIR. The various LDs may bespatially multiplexed, polarization multiplexed, wavelength multiplexed,or a combination of these multiplexing methods. Also, the LDs may befiber pig-tailed or have one or more lenses on the output to collimateor focus the light. Another advantage of LDs is that they may bepackaged compactly and may have a spatially coherent beam output.Moreover, tunable LDs that can tune over a range of wavelengths are alsoavailable. The tuning may be done by varying the temperature, orelectrical current may be used in particular structures such asdistributed Bragg reflector LDs. In another embodiment, external cavityLDs may be used that have a tuning element, such as a fiber grating or abulk grating, in the external cavity.

In another embodiment, super-luminescent laser diodes may provide higherpower as well as broad bandwidth. An SLD is typically an edge emittingsemiconductor light source based on super-luminescence (e.g., this couldbe amplified spontaneous emission). SLDs combine the higher power andbrightness of LDs with the low coherence of conventional LEDs, and theemission band for SLD's may be 5 nm to 100 nm wide, preferably in the 60nm to 100 nm range. Although currently SLDs are commercially availablein the wavelength range of approximately 400 nm to 1700 nm, SLDs couldand may in the future be made that cover a broader region of the SWIR.

In yet another embodiment, high power LDs for either direct excitationor to pump fiber lasers and SC light sources may be constructed usingone or more laser diode bar stacks. FIG. 19 shows an example of theblock diagram 1900 or building blocks for constructing the high powerLDs. In this embodiment, one or more diode bar stacks 1901 may be used,where the diode bar stack may be an array of several single emitter LDs.Since the fast axis (e.g., vertical direction) may be nearly diffractionlimited while the slow-axis (e.g., horizontal axis) may be far fromdiffraction limited, different collimators 1902 may be used for the twoaxes.

Then, the brightness may be increased by spatially combining the beamsfrom multiple stacks 1903. The combiner may include spatialinterleaving, it may include wavelength multiplexing, or it may involvea combination of the two. Different spatial interleaving schemes may beused, such as using an array of prisms or mirrors with spacers to bendone array of beams into the beam path of the other. In anotherembodiment, segmented mirrors with alternate high-reflection andanti-reflection coatings may be used. Moreover, the brightness may beincreased by polarization beam combining 1904 the two orthogonalpolarizations, such as by using a polarization beam splitter. In aparticular embodiment, the output may then be focused or coupled into alarge diameter core fiber. As an example, typical dimensions for thelarge diameter core fiber range from diameters of approximately 100microns to 400 microns or more. Alternatively or in addition, a custombeam shaping module 1905 may be used, depending on the particularapplication. For example, the output of the high power LD may be useddirectly 1906, or it may be fiber coupled 1907 to combine, integrate, ortransport the high power LD energy. These high power LDs may grow inimportance because the LD powers can rapidly scale up. For example,instead of the power being limited by the power available from a singleemitter, the power may increase in multiples depending on the number ofdiodes multiplexed and the size of the large diameter fiber. AlthoughFIG. 19 is shown as one embodiment, some or all of the elements may beused in a high power LD, or addition elements may also be used.

SWIR Super-Continuum Lasers

Each of the light sources described above have particular strengths, butthey also may have limitations. For example, there is typically atrade-off between wavelength range and power output. Also, sources suchas lamps, thermal sources, and LEDs produce incoherent beams that may bedifficult to focus to a small area and may have difficulty propagatingfor long distances. An alternative source that may overcome some ofthese limitations is an SC light source. Some of the advantages of theSC source may include high power and intensity, wide bandwidth,spatially coherent beam that can propagate nearly transform limited overlong distances, and easy compatibility with fiber delivery.

Supercontinuum lasers may combine the broadband attributes of lamps withthe spatial coherence and high brightness of lasers. By exploiting amodulational instability initiated supercontinuum (SC) mechanism, anall-fiber-integrated SC laser with no moving parts may be built usingcommercial-off-the-shelf (COTS) components. Moreover, the fiber laserarchitecture may be a platform where SC in the visible,near-infrared/SWIR, or mid-IR can be generated by appropriate selectionof the amplifier technology and the SC generation fiber. But untilrecently, SC lasers were used primarily in laboratory settings sincetypically large, table-top, mode-locked lasers were used to pumpnonlinear media such as optical fibers to generate SC light. However,those large pump lasers may now be replaced with diode lasers and fiberamplifiers that gained maturity in the telecommunications industry.

In one embodiment, an all-fiber-integrated, high-powered SC light source2000 may be elegant for its simplicity (FIG. 20). The light may be firstgenerated from a seed laser diode 2001. For example, the seed LD 2001may be a distributed feedback laser diode with a wavelength near 1542 nmor 1550 nm, with approximately 0.5-2.0 ns pulsed output, and with apulse repetition rate between one kilohertz to about 100 MHz or more.The output from the seed laser diode may then be amplified in amultiple-stage fiber amplifier 2002 comprising one or more gain fibersegments. In a particular embodiment, the first stage pre-amplifier 2003may be designed for optimal noise performance. For example, thepre-amplifier 2003 may be a standard erbium-doped fiber amplifier or anerbium/ytterbium doped cladding pumped fiber amplifier. Betweenamplifier stages 2003 and 2006, it may be advantageous to use band-passfilters 2004 to block amplified spontaneous emission and isolators 2005to prevent spurious reflections. Then, the power amplifier stage 2006may use a cladding-pumped fiber amplifier that may be optimized tominimize nonlinear distortion. The power amplifier fiber 2006 may alsobe an erbium-doped fiber amplifier, if only low or moderate power levelsare to be generated.

The SC generation 2007 may occur in the relatively short lengths offiber that follow the pump laser. Exemplary SC fiber lengths may rangefrom a few millimeters to 100 m or more. In one embodiment, the SCgeneration may occur in a first fiber 2008 where themodulational-instability initiated pulse break-up occurs primarily,followed by a second fiber 2009 where the SC generation and spectralbroadening occurs primarily.

In one embodiment, one or two meters of standard single-mode fiber (SMF)after the power amplifier stage may be followed by several meters of SCgeneration fiber. For this example, in the SMF the peak power may beseveral kilowatts and the pump light may fall in the anomalousgroup-velocity dispersion regime—often called the soliton regime. Forhigh peak powers in the anomalous dispersion regime, the nanosecondpulses may be unstable due to a phenomenon known as modulationalinstability, which is basically parametric amplification in which thefiber nonlinearity helps to phase match the pulses. As a consequence,the nanosecond pump pulses may be broken into many shorter pulses as themodulational instability tries to form soliton pulses from thequasi-continuous-wave background. Although the laser diode andamplification process starts with approximately nanosecond-long pulses,modulational instability in the short length of SMF fiber may formapproximately 0.5 ps to several-picosecond-long pulses with highintensity. Thus, the few meters of SMF fiber may result in an outputsimilar to that produced by mode-locked lasers, except in a much simplerand cost-effective manner.

The short pulses created through modulational instability may then becoupled into a nonlinear fiber for SC generation. The nonlinearmechanisms leading to broadband SC may include four-wave mixing orself-phase modulation along with the optical Raman effect. Since theRaman effect is self-phase-matched and shifts light to longerwavelengths by emission of optical photons, the SC may spread to longerwavelengths very efficiently. The short-wavelength edge may arise fromfour-wave mixing, and often times the short wavelength edge may belimited by increasing group-velocity dispersion in the fiber. In manyinstances, if the particular fiber used has sufficient peak power and SCfiber length, the SC generation process may fill the long-wavelengthedge up to the transmission window.

Mature fiber amplifiers for the power amplifier stage 2006 includeytterbium-doped fibers (near 1060 nm), erbium-doped fibers (near 1550nm), erbium/ytterbium-doped fibers (near 1550 nm), or thulium-dopedfibers (near 2000 nm). In various embodiments, candidates for SC fiber2009 include fused silica fibers (for generating SC between 0.8-2.7 μm),mid-IR fibers such as fluorides, chalcogenides, or tellurites (forgenerating SC out to 4.5 μm or longer), photonic crystal fibers (forgenerating SC between 0.4-1.7 μm), or combinations of these fibers.Therefore, by selecting the appropriate fiber-amplifier doping for 2006and nonlinear fiber 2009, SC may be generated in the visible,near-IR/SWIR, or mid-IR wavelength region.

The configuration 2000 of FIG. 20 is just one particular example, andother configurations can be used and are intended to be covered by thisdisclosure. For example, further gain stages may be used, and differenttypes of lossy elements or fiber taps may be used between the amplifierstages. In another embodiment, the SC generation may occur partially inthe amplifier fiber and in the pig-tails from the pump combiner or otherelements. In yet another embodiment, polarization maintaining fibers maybe used, and a polarizer may also be used to enhance the polarizationcontrast between amplifier stages. Also, not discussed in detail aremany accessories that may accompany this set-up, such as driverelectronics, pump laser diodes, safety shut-offs, and thermal managementand packaging.

One example of the SC laser that operates in the SWIR is illustrated inFIG. 21. This SWIR SC source 2100 produces an output of up toapproximately 5 W over a spectral range of about 1.5-2.4 microns, andthis particular laser is made out of polarization maintainingcomponents. The seed laser 2101 is a distributed feedback laseroperating near 1542 nm producing approximately 0.5 nsec pulses at anabout 8 MHz repetition rate. The pre-amplifier 2102 is forward pumpedand uses about 2 m length of erbium/ytterbium cladding pumped fiber 2103(often also called dual-core fiber) with an inner core diameter of 12microns and outer core diameter of 130 microns. The pre-amplifier gainfiber 2103 is pumped using a 10 W laser diode near 940 nm 2105 that iscoupled in using a fiber combiner 2104.

In this particular 5 W unit, the mid-stage between amplifier stages 2102and 2106 comprises an isolator 2107, a band-pass filter 2108, apolarizer 2109 and a fiber tap 2110. The power amplifier 2106 uses anapproximately 4 m length of the 12/130 micron erbium/ytterbium dopedfiber 2111 that is counter-propagating pumped using one or more 30 Wlaser diodes near 940 nm 2112 coupled in through a combiner 2113. Anapproximately 1-2 m length of the combiner pig-tail helps to initiatethe SC process, and then a length of PM-1550 fiber 2115 (polarizationmaintaining, single-mode, fused silica fiber optimized for 1550 nm) isspliced 2114 to the combiner output.

If an approximately 10 m length of output fiber is used, then theresulting output spectrum 2200 is shown in FIG. 22. The details of theoutput spectrum 2200 depend on the peak power into the fiber, the fiberlength, and properties of the fiber such as length and core size, aswell as the zero dispersion wavelength and the dispersion properties.For example, if a shorter length of fiber is used, then the spectrumactually reaches to longer wavelengths (e.g., a 2 m length of SC fiberbroadens the spectrum to about 2500 nm). Also, if extra-dry fibers areused with less O—H content, then the wavelength edge may also reach to alonger wavelength. To generate more spectrum toward the shorterwavelengths, the pump wavelength (in this case around 1542 nm) should beclose to the zero dispersion wavelength in the fiber. For example, byusing a dispersion shifted fiber or so-called non-zero dispersionshifted fiber, the short wavelength edge may shift to shorterwavelengths.

Although one particular example of a 5 W SWIR-SC has been described,different components, different fibers, and different configurations mayalso be used consistent with this disclosure. For instance, anotherembodiment of the similar configuration 2100 in FIG. 21 may be used togenerate high powered SC between approximately 1060 nm and 1800 nm. Forthis embodiment, the seed laser 2101 may be a distributed feedback laserdiode near 1064 nm, the pre-amplifier gain fiber 2103 may be aytterbium-doped fiber amplifier with 10/125 microns dimensions, and thepump laser 2105 may be a 10 W laser diode near 915 nm. A mode fieldadapter may be included in the mid-stage, in addition to the isolator2107, band pass filter 2108, polarizer 2109 and tap 2110. The gain fiber2111 in the power amplifier may be a ytterbium-doped fiber with 25/400microns dimension of about 20 m length. The pump 2112 for the poweramplifier may be up to six pump diodes providing 30 W each near 915 nm.For this much pump power, the output power in the SC may be as high as50 W or more.

In an alternate embodiment, it may be desirous to generate high powerSWIR SC over 1.4-1.8 microns and separately 2-2.5 microns (the windowbetween 1.8 and 2 microns may be less important due to the strong waterand atmospheric absorption). For example, the top SC source of FIG. 23can lead to bandwidths ranging from about 1400 nm to 1800 nm or broader,while the lower SC source of FIG. 23 can lead to bandwidths ranging fromabout 1900 nm to 2500 nm or broader. Since these wavelength ranges areshorter than about 2500 nm, the SC fiber can be based on fused silicafiber. Exemplary SC fibers include standard single-mode fiber (SMF),high-nonlinearity fiber, high-NA fiber, dispersion shifted fiber,dispersion compensating fiber, and photonic crystal fibers.Non-fused-silica fibers can also be used for SC generation, includingchalcogenides, fluorides, ZBLAN, tellurites, and germanium oxide fibers.

In one embodiment, the top of FIG. 23 illustrates a block diagram for anSC source 2300 capable of generating light between approximately 1400 nmand 1800 nm or broader. As an example, a pump fiber laser similar toFIG. 21 can be used as the input to a SC fiber 2309. The seed laserdiode 2301 can comprise a DFB laser that generates, for example, severalmilliwatts of power around 1542 nm or 1553 nm. The fiber pre-amplifier2302 can comprise an erbium-doped fiber amplifier or an erbium/ytterbiumdoped double clad fiber. In this example a mid-stage amplifier 2303 canbe used, which can comprise an erbium/ytterbium doped double-clad fiber.A bandpass filter 2305 and isolator 2306 may be used between thepre-amplifier 2302 and mid-stage amplifier 2303. The power amplifierstage 2304 can comprise a larger core size erbium/ytterbium dopeddouble-clad fiber, and another bandpass filter 2307 and isolator 2308can be used before the power amplifier 2304. The output of the poweramplifier can be coupled to the SC fiber 2309 to generate the SC output2310. This is just one exemplary configuration for an SC source, andother configurations or elements may be used consistent with thisdisclosure.

In yet another embodiment, the bottom of FIG. 23 illustrates a blockdiagram for an SC source 2350 capable of generating light, for example,between approximately 1900 nm and 2500 nm or broader. As an example, theseed laser diode 2351 can comprise a DFB or DBR laser that generates,for example, several milliwatts of power around 1542 nm or 1553 nm. Thefiber pre-amplifier 2352 can comprise an erbium-doped fiber amplifier oran erbium/ytterbium doped double-clad fiber. In this example a mid-stageamplifier 2353 can be used, which can comprise an erbium/ytterbium dopeddouble-clad fiber. A bandpass filter 2355 and isolator 2356 may be usedbetween the pre-amplifier 2352 and mid-stage amplifier 2353. The poweramplifier stage 2354 can comprise a thulium doped double-clad fiber, andanother isolator 2357 can be used before the power amplifier 2354. Notethat the output of the mid-stage amplifier 2353 can be approximatelynear 1542 nm, while the thulium-doped fiber amplifier 2354 can amplifywavelengths longer than approximately 1900 nm and out to about 2100 nm.Therefore, for this configuration wavelength shifting may be requiredbetween 2353 and 2354. In one embodiment, the wavelength shifting can beaccomplished using a length of standard single-mode fiber 2358, whichcan have a length between approximately 5 m and 50 m, for example. Theoutput of the power amplifier 2354 can be coupled to the SC fiber 2359to generate the SC output 2360. This is just one exemplary configurationfor an SC source, and other configurations or elements can be usedconsistent with this disclosure. For example, the various amplifierstages can comprise different amplifier types, such as erbium dopedfibers, ytterbium doped fibers, erbium/ytterbium co-doped fibers andthulium doped fibers. One advantage of the SC lasers illustrated inFIGS. 20, 21, and 23 are that they may use all-fiber components, so thatthe SC laser can be all-fiber, monolithically integrated with no movingparts. The all-integrated configuration can consequently be robust andreliable.

FIGS. 20, 21 and 23 are examples of SC light sources that may beadvantageously used for SWIR light generation in various active remotesensing and hyper-spectral imaging applications. However, many otherversions of the SC light sources may also be made that are intended toalso be covered by this disclosure. For example, the SC generation fibercould be pumped by a mode-locked laser, a gain-switched semiconductorlaser, an optically pumped semiconductor laser, a solid state laser,other fiber lasers, or a combination of these types of lasers. Also,rather than using a fiber for SC generation, either a liquid or a gascell might be used as the nonlinear medium in which the spectrum is tobe broadened.

Even within the all-fiber versions illustrated such as in FIG. 21,different configurations could be used consistent with the disclosure.In an alternate embodiment, it may be desirous to have a lower costversion of the SWIR SC laser of FIG. 21. One way to lower the cost couldbe to use a single stage of optical amplification, rather than twostages, which may be feasible if lower output power is required or thegain fiber is optimized. For example, the pre-amplifier stage 2102 mightbe removed, along with at least some of the mid-stage elements. In yetanother embodiment, the gain fiber could be double passed to emulate atwo stage amplifier. In this example, the pre-amplifier stage 2102 mightbe removed, and perhaps also some of the mid-stage elements. A mirror orfiber grating reflector could be placed after the power amplifier stage2106 that may preferentially reflect light near the wavelength of theseed laser 2101. If the mirror or fiber grating reflector can transmitthe pump light near 940 nm, then this could also be used instead of thepump combiner 2113 to bring in the pump light 2112. The SC fiber 2115could be placed between the seed laser 2101 and the power amplifierstage 2106 (SC is only generated after the second pass through theamplifier, since the power level may be sufficiently high at that time).In addition, an output coupler may be placed between the seed laserdiode 2101 and the SC fiber, which now may be in front of the poweramplifier 2106. In a particular embodiment, the output coupler could bea power coupler or divider, a dichroic coupler (e.g., passing seed laserwavelength but outputting the SC wavelengths), or a wavelength divisionmultiplexer coupler. This is just one further example, but a myriad ofother combinations of components and architectures could also be usedfor SC light sources to generate SWIR light that are intended to becovered by this disclosure.

FIG. 24 illustrates a flowchart 2400 of a smart manufacturing process.The manufacturing process 2401 may have as input the process feed 2402and result in a process output 2403. A process controller 2404 may atleast partially control the manufacturing process 2401, and thecontroller 2404 may receive inputs from the closed loop control (processparameters) 2405 as well as the on-line monitoring of process parameters2406. The feedback loops in the process could refine the manufacturingprocess 2401 and improve the quality of the process output 2403. Theseare particular embodiments of the use of near-infrared or SWIRspectroscopy in the PAT of the pharmaceutical industry, but othervariations, combinations, and methods may also be used and are intendedto be covered by this disclosure, such as use in manufacture of plasticsor food industry goods.

The discussion thus far has centered on use of near-infrared or SWIRspectroscopy in applications such as identification of counterfeitdrugs, detection of illicit drugs, and pharmaceutical process control.Although drugs and pharmaceuticals are one example, many other fieldsand applications may also benefit from the use of near infrared or SWIRspectroscopy, and these may also be implemented without departing fromthe scope of this disclosure. As just another example, near-infrared orSWIR spectroscopy may also be used as an analytic tool for food qualityand safety control. Applications in food safety and quality assessmentinclude contaminant detection, defect identification, constituentanalysis, and quality evaluation. The techniques described in thisdisclosure are particularly valuable when non-destructive testing isdesired at stand-off or remote distances.

In yet another embodiment, near-infrared or SWIR spectroscopy may beused for the assessment of fruit and vegetable quality. Most commercialquality classification systems for fruit and vegetables are based onexternal features of the product, such as shape, color, size, weight andblemishes. However, the external appearance of most fruit is generallynot an accurate guide to the internal eating quality of the fruit. As anexample, for avocado fruit the external color is not a maturitycharacteristic, and its smell is too weak and appears later in itsmaturity stage. Analysis of the near-infrared or SWIR absorption spectramay provide qualitative and quantitative determination of manyconstituents and properties of horticulture produce, including oil,water, protein, pH, acidity, firmness, and soluble solids content ortotal soluble solids of fresh fruits. For example, near-infraredabsorbance spectra may be obtained in diffusion reflectance mode for aseries of whole ‘Hass’ avocado fruit. Four oil absorption bands are near2200-2400 nm (CH₂ stretch bend and combinations), with weaker absorptionaround 750 nm, 1200 nm, and 900-930 nm ranges. On the other hand, near1300-1750 nm range may be useful for determining the protein and oilcontent. The 900-920 nm absorbance band may be useful for sugardetermination. Although described in the context of grains, fruits, andvegetables, the near-infrared or SWIR spectroscopy may also be valuablefor other food quality control and assessment, such as measuring theproperties of meats. These and other applications also fall within thescope of this disclosure.

Described herein are just some examples of the beneficial use ofnear-infrared or SWIR lasers for active remote sensing or hyper-spectralimaging. However, many other spectroscopy and identification procedurescan use the near-infrared or SWIR light consistent with this disclosureand are intended to be covered by the disclosure. As one example, thefiber-based super-continuum lasers may have a pulsed output with pulsedurations of approximately 0.5-2 nsec and pulse repetition rates ofseveral Megahertz. Therefore, the active remote sensing orhyper-spectral imaging applications may also be combined with LIDAR-typeapplications. Namely, the distance or time axis can be added to theinformation based on time-of-flight measurements. For this type ofinformation to be used, the detection system would also have to betime-gated to be able to measure the time difference between the pulsessent and the pulses received. By calculating the round-trip time for thesignal, the distance of the object may be judged. In another embodiment,GPS (global positioning system) information may be added, so the activeremote sensing or hyper-spectral imagery would also have a location tagon the data. Moreover, the active remote sensing or hyper-spectralimaging information could also be combined with two-dimensional orthree-dimensional images to provide a physical picture as well as achemical composition identification of the materials. These are justsome modifications of the active remote sensing or hyper-spectralimaging system described in this disclosure, but other techniques mayalso be added or combinations of these techniques may be added, andthese are also intended to be covered by this disclosure.

Although the present disclosure has been described in severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present disclosure encompass suchchanges, variations, alterations, transformations, and modifications asfalling within the spirit and scope of the appended claims.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the disclosure. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the disclosure.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the disclosure. While variousembodiments may have been described as providing advantages or beingpreferred over other embodiments with respect to one or more desiredcharacteristics, as one skilled in the art is aware, one or morecharacteristics may be compromised to achieve desired system attributes,which depend on the specific application and implementation. Theseattributes include, but are not limited to: cost, strength, durability,life cycle cost, marketability, appearance, packaging, size,serviceability, weight, manufacturability, ease of assembly, etc. Theembodiments described herein that are described as less desirable thanother embodiments or prior art implementations with respect to one ormore characteristics are not outside the scope of the disclosure and maybe desirable for particular applications.

What is claimed is:
 1. A measurement system comprising: a light sourceconfigured to generate an output optical beam, comprising: a pluralityof semiconductor sources configured to generate an input optical beam; amultiplexer configured to receive at least a portion of the inputoptical beam and to form an intermediate optical beam; one or morefibers configured to receive at least a portion of the intermediateoptical beam and to form the output optical beam; wherein at least aportion of the one or more fibers comprises a fused silica fiber havinga core diameter less than 400 microns; wherein the output optical beamcomprises one or more optical wavelengths, at least a portion of whichare between 700 nanometers and 2500 nanometers; and wherein the outputoptical beam has a bandwidth of at least 10 nanometers; a measurementapparatus configured to receive a received portion of the output opticalbeam and to deliver a delivered portion of the output optical beam to asample, wherein the delivered portion of the output optical beam isconfigured to generate a spectroscopy output beam from the sample; and areceiver configured to receive at least a portion of the spectroscopyoutput beam having a bandwidth of at least 10 nanometers and to processthe at least a portion of the spectroscopy output beam to generate anoutput signal; wherein the light source and the receiver are remote fromthe sample, wherein the sample comprises plastics or food industrygoods, and wherein the measurement system performs non-destructivequality control or constitutive analysis.
 2. The system of claim 1,wherein the measurement apparatus is a stand-off detection apparatus,and the spectroscopy output beam is based at least in part on diffusereflection from the sample.
 3. The system of claim 1, wherein themeasurement system is used for on-line process control.
 4. The system ofclaim 1, wherein the output signal at least in part determines the sugarcontent in solid food industry goods.
 5. The system of claim 1, whereinthe output signal is based on a chemical composition of the sample.
 6. Ameasurement system comprising: a light source configured to generate anoutput optical beam, comprising: a plurality of semiconductor sourcesconfigured to generate an input optical beam; a multiplexer configuredto receive at least a portion of the input optical beam and to form anintermediate optical beam; one or more fibers configured to receive atleast a portion of the intermediate optical beam and to form the outputoptical beam; wherein at least a portion of the one or more fiberscomprises a fused silica fiber having a core diameter less than 400microns; wherein the output optical beam comprises one or more opticalwavelengths, at least a portion of which are between 700 nanometers and2500 nanometers; and wherein the output optical beam has a bandwidth ofat least 10 nanometers; a measurement apparatus configured to receive areceived portion of the output optical beam and to deliver a deliveredportion of the output optical beam to a sample, wherein the deliveredportion of the output optical beam is configured to generate aspectroscopy output beam from the sample; and a receiver configured toreceive at least a portion of the spectroscopy output beam having abandwidth of at least 10 nanometers and to process the at least aportion of the spectroscopy output beam to generate an output signal;wherein the output signal is based on a chemical composition of thesample; wherein the spectroscopy output beam comprises spectral featuresof hydrocarbons or organic compounds; and wherein the light source andthe receiver are remote from the sample, and wherein the samplecomprises plastics or food industry goods.
 7. The system of claim 6,wherein the light source comprises a super-continuum laser.
 8. Thesystem of claim 6, wherein the measurement apparatus is a stand-offdetection apparatus, and the spectroscopy output beam is based at leastin part on diffuse reflection from the sample.
 9. The system of claim 6,wherein the measurement system is used for non-destructive testing ofthe sample.
 10. The system of claim 6, wherein the measurement system isused for quality control or constitutive analysis of the sample.
 11. Thesystem of claim 6, wherein the receiver comprises a wavelength tunabledetection system.
 12. The system of claim 6, wherein the measurementsystem is used for on-line process control.
 13. The system of claim 1,wherein the output signal corresponds to overtone or combinational bandsof the chemical composition of the sample.
 14. The system of claim 6,wherein the output signal at least in part determines the sugar contentin solid food industry goods.
 15. A method of measuring, comprising:generating an output optical beam, comprising: generating an inputoptical beam from a plurality of semiconductor sources; multiplexing atleast a portion of the input optical beam and forming an intermediateoptical beam; and guiding at least a portion of the intermediate opticalbeam using one or more fibers including a fused silica fiber having acore diameter less than 400 microns; and broadening a spectrum of atleast a portion of the intermediate optical beam to at least 10nanometers to form the output optical beam with an output beam broadenedspectrum, wherein the output optical beam comprises one or more opticalwavelengths between 700 nanometers and 2500 nanometers; receiving areceived portion of the output optical beam and delivering a deliveredportion of the output optical beam to a sample comprising plastics orfood industry goods located remotely from the generated output opticalbeam; generating a spectroscopy output beam having a bandwidth of atleast 10 nanometers from the sample, wherein the spectroscopy outputbeam comprises spectral features based on a chemical composition of thesample; receiving at least a portion of the spectroscopy output beam;and processing the at least a portion of the spectroscopy output beamand generating an output signal; wherein the measuring method is usedfor non-destructive quality control or constitutive analysis.
 16. Themethod of claim 15, wherein the output signal at least in partdetermines the sugar content in solid food industry goods.
 17. Themethod of claim 15, wherein the measuring method is used for on-lineprocess control.